Vacuum processing operations on substrates, such as deposition of thin films, often require control over substrate temperature. Deposition operations include sputtering, evaporation, and chemical vapor deposition. Related operations such as cleaning, planarizing, annealing, and etching can also require substrate temperature controls.
Some operations require the addition of heat, and others require excess heat to be removed. Also, certain combinations of operations require both heating and cooling in various orders. Heating can be accomplished with radiant heaters that focus heat energy on substrate surfaces. Cooling is often accomplished by removing the substrates from the evacuated environment. However, with these limited controls, constant substrate temperatures are difficult to maintain and temperature transitions can be time consuming.
Better control over substrate temperature has been achieved by transferring heat through substrate supports or chucks. Ordinarily, conduction in a vacuum environment between the substrate and its support is highly inefficient. However, more efficient transfers of heat are possible by filling gaps between the substrate and its support with a gas that is compatible with the vacuum processing environment.
For example, U.S. Pat. No. 4,909,314 to Lamont, Jr. discloses a substrate support that also functions as a temperature conditioner such as a heating unit or a cooling unit. The substrate is clamped to the support, and a cavity between the substrate and the support is filled with a gas such as argon. The gas, which is maintained at a pressure above that of the vacuum processing environment but well below atmospheric pressure, transfers heat by convection between the substrate and substrate support.
U.S. Pat. No. 4,949,783 to Lakios et al. discloses a similar substrate support arranged as a cooling unit. However, instead of merely conducting gas into a cavity between the substrate and its support, the gas is circulated into and out of the cavity to carry away excess heat. The cooling unit has additional areas exposed to the circulating gas to extract the heat from the gas.
Although the known substrate supports can be arranged to operate effectively as either heating units or cooling units, some combinations of vacuum processing operations require both heating and cooling to achieve desired temperature patterns. Substrate supports arranged for heating are slow to cool, and substrate supports arranged for cooling provide no means for heating. Thus, controlled variations in substrate temperatures are limited to either increases or decreases in temperature, and constant substrate temperatures are difficult to maintain in processing environments that also involve transfers of heat.